Concept

Cosmic Microwave Background

Intro

Point a sensitive microwave antenna at any patch of empty sky, day or night, in any direction, and you will pick up a faint hiss. It is not from the atmosphere, not from the galaxy, not from any star. It is the light of the universe itself, cooled by expansion to 2.7 degrees above absolute zero.

That light left its source about 380,000 years after the beginning. Before then the universe was an opaque glowing fog, too hot for electrons to settle into atoms, so light could not travel far without colliding with something. When it cooled past roughly 3,000 degrees, electrons and nuclei combined into neutral hydrogen, the fog cleared in an instant, and the light that had been trapped streamed out unobstructed. It has been travelling ever since.

Two things about it are remarkable.

First, it was predicted before it was found. Working from the idea of a hot early universe, physicists in the 1940s calculated that such an afterglow should exist and estimated its temperature within a few degrees of what was eventually measured.

Second, it is almost perfectly smooth, but not quite. The temperature is the same in every direction to about one part in a hundred thousand. Those tiny remaining ripples are the seeds from which every galaxy grew, and their exact pattern encodes the composition, geometry, and age of the cosmos with startling precision.

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In full

The cosmic microwave background is the relic radiation of the hot early universe, released at recombination when the primordial plasma cooled sufficiently for electrons and protons to form neutral hydrogen, ending Thomson scattering and rendering the universe transparent. This occurred at redshift z ≈ 1100, roughly 380,000 years after the beginning, and the sphere from which the observed photons last scattered is called the surface of last scattering.

The radiation has a present-day temperature of 2.725 K and a spectrum that is the closest approach to a perfect blackbody ever measured in nature, a fact established by the FIRAS instrument aboard COBE. Superimposed on that near-uniformity are anisotropies of order one part in 100,000, first detected by COBE's DMR instrument in 1992 and mapped in progressively finer detail by WMAP and Planck.

The statistical structure of those anisotropies, expressed as the angular power spectrum, exhibits a series of acoustic peaks generated by the same pressure waves that leave the Baryon Acoustic Oscillations imprint in the galaxy distribution. The positions and relative heights of those peaks are the single richest source of cosmological parameters available: the spatial geometry, the baryon density, the dark-matter density, and the expansion rate are all read from them.

The phenomenon

Prediction. Ralph Alpher and Robert Herman, working from George Gamow's hot Big Bang model, predicted in 1948 that the universe should be pervaded by relic radiation of a few degrees Kelvin. The prediction was largely forgotten for over a decade.

Discovery. In 1964 and 1965, Arno Penzias and Robert Wilson, using a horn antenna at Bell Labs in Holmdel, New Jersey, found a persistent excess noise they could not eliminate. They checked the equipment exhaustively and evicted a pair of nesting pigeons before concluding the signal was real and came from every direction. Robert Dicke, Jim Peebles, Peter Roll and David Wilkinson at Princeton, who were preparing to search for exactly this radiation, supplied the interpretation in a companion paper. Penzias and Wilson received the Nobel Prize in 1978.

The blackbody spectrum. COBE's FIRAS instrument returned a spectrum matching a 2.725 K blackbody so exactly that the error bars are smaller than the width of the plotted curve. This is decisive: a blackbody of that quality requires a period of full thermal equilibrium, which is to say a hot dense phase. No scattered-starlight or tired-light alternative reproduces it.

Anisotropies. COBE's DMR found the first temperature fluctuations in 1992; John Mather and George Smoot shared the 2006 Nobel Prize for the FIRAS and DMR results. WMAP (2001-2010) and Planck (2009-2013) mapped them with successively greater angular resolution.

What the acoustic peaks encode.

• The angular position of the first peak fixes the spatial geometry. It lands where flat space predicts, giving a total density parameter indistinguishable from one.
• The relative heights of odd and even peaks fix the baryon density, because baryons load the oscillating fluid and deepen compressions relative to rarefactions.
• The third peak constrains the dark-matter density.
• The damping tail at small scales reflects the finite thickness of the last-scattering surface.

Polarization, and a lesson in scientific honesty. The CMB is slightly polarized. E-mode polarization was detected by DASI in 2002 and is now well mapped. B-modes produced by gravitational lensing have been detected. Primordial B-modes, the signature of inflationary gravitational waves, have not. The BICEP2 collaboration announced such a detection in 2014; joint analysis with Planck showed the signal was consistent with polarized galactic dust, and the claim was withdrawn. The episode is worth citing whenever someone charges that cosmologists will not abandon a favoured result.

The design inference

1. A prediction made, then found, at the stated temperature. Alpher and Herman derived the existence and approximate temperature of the background in 1948 from a model of a hot origin, and it was found seventeen years later by two men who were not looking for it and initially thought it was a fault in their antenna. Accidental confirmation of a prior theoretical prediction is the strongest form scientific evidence takes, and what it confirms is a universe with a definite hot beginning.

2. The most perfect blackbody in nature. The FIRAS spectrum requires that the entire observable universe was once in thorough thermal equilibrium. That is a direct measurement of an early state, and it forecloses the steady-state and eternal-universe cosmologies that dominated the field before it. The universe had an origin, and this is the photograph of it.

3. Smooth to one part in 100,000, and that number is exact enough to matter. Had the primordial fluctuations been appreciably larger, matter would have collapsed into black holes rather than galaxies. Appreciably smaller, and no structure would have formed at all: no galaxies, no stars, no planets, no chemistry. The amplitude sits in the narrow band that yields a universe containing observers, and the CMB is the instrument that measures it. See Fine-Tuning Argument and Anthropic Principle.

4. Flatness measured, not assumed. The first acoustic peak lands where spatial flatness predicts. A universe balanced that precisely between recollapse and runaway dispersal is one of the most striking specified quantities in physics, and it is read directly off the sky.

5. The heavens are legible. The whole enterprise depends on the cosmos having preserved, for 13.8 billion years, a coherent record of its own infancy, and on that record being decodable by rational creatures using mathematics developed for other purposes. The Mathematical Intelligibility of Nature is not a background assumption here; it is the working condition of the entire measurement.

Atheist responses and rebuttals

1. "The CMB is evidence for the Big Bang, not for God."

It is evidence for a universe with a hot, dense origin, exact physical law, and initial conditions falling in the narrow range that permits structure and observers. Each of those is what theism leads one to expect and none is explained by naturalism, which must accept them as brute. The theist is not filling a gap; he is accounting for the settled result. See God of the Gaps.

2. "Inflation explains the uniformity and the fluctuation spectrum."

Inflation is an excellent mechanism and the codex has no quarrel with it. It requires an inflaton field with a specific potential, specific initial conditions, and a graceful exit, all unexplained by the mechanism itself. Explaining the smoothness by positing a finely conditioned field pushes the specification back one level rather than removing it. It also does not evade the Borde-Guth-Vilenkin Theorem, which applies to inflationary spacetimes directly.

3. "The Hubble tension shows cosmologists do not really know what they are doing."

The tension is real and unresolved: the expansion rate inferred from the CMB and BAO disagrees with the rate measured from Cepheid-calibrated supernovae by around five standard deviations. What it demonstrates is a field measuring the same quantity two independent ways precisely enough for a small discrepancy to be visible and taken seriously. Nothing in the tension touches the hot origin, the blackbody spectrum, or the acoustic peaks. See Age of the Universe.

4. "Tired light or a steady-state model could produce the background."

Neither survives the FIRAS spectrum. Scattering mechanisms that would thermalize starlight into so exact a blackbody would also blur distant sources, which is not observed, and the steady-state model has no epoch capable of producing thermal equilibrium across the observable universe. Fred Hoyle, who coined "Big Bang" derisively and defended steady state to the end, lost the argument on this evidence.

Biblical anticipation and theological resonance

The codex does not read technical cosmology out of Hebrew poetry. What Scripture supplies is the frame in which these findings are unsurprising.

• Light before luminaries. Genesis 1:3 has light called into being before the sun and stars are appointed on the fourth day. Whatever one's reading of the days, an ancient text placing primordial light prior to the formation of stellar bodies sits comfortably beside a cosmology in which the universe was radiation-dominated long before the first star. See Light of Day 1, Christological Reading.
• A beginning that was hot and bright. Genesis 1:1 asserts an absolute origin. The CMB is the direct observational trace of that origin's thermal state.
• The heavens declare. Psalm 19:1-4 describes a testimony without speech, whose line has gone out through all the earth. Radiation arriving from every direction, carrying the record of the universe's infancy, is a fitting literal companion to the image.
• Stretched out like a curtain. Isaiah 40:22 and Isaiah 42:5 describe the heavens stretched out. The expansion that cooled this radiation from 3,000 K to 2.7 K is that stretching in physical terms, and the redshift of these photons is its measure.

Apologetic deployment

• Lead with the accidental discovery. Predicted in 1948, found in 1965 by two engineers chasing a noise problem who cleaned out pigeons before accepting the result. The story is memorable, verifiable, and makes the point that the evidence was not manufactured to fit a conclusion.
• Use FIRAS against eternal-universe models. The most perfect blackbody in nature is a direct measurement of a hot early state and is the fact that ended the steady-state debate.
• Use the one-part-in-100,000 figure for fine-tuning. It is concrete, easy to state, and its consequences (black holes if larger, no structure if smaller) are vivid.
• Cite BICEP2 when accused of dogmatism. A major collaboration announced a Nobel-calibre detection, the community checked it, the signal proved to be galactic dust, and the claim was withdrawn. That is the process working.
• Pair with Baryon Acoustic Oscillations, which is the same acoustic physics in the matter distribution, calibrated here and applied there.

See also

• Baryon Acoustic Oscillations, the same acoustic physics imprinted in galaxies
• Big Bang, the framework the CMB confirms
• Expansion of the Universe, the process that cooled this radiation to 2.7 K
• Age of the Universe, derived largely from the CMB power spectrum
• Dark Energy, constrained by CMB data in combination with distance measures
• Borde-Guth-Vilenkin Theorem, the theorem that carries a hot dense phase to a past boundary
• Kalam Cosmological Argument, the argument this evidence serves
• Fine-Tuning Argument, Anthropic Principle, where the flatness and fluctuation amplitude land
• Stellar Nucleosynthesis, the light-element line of evidence for the same early epoch
• Mathematical Intelligibility of Nature, the condition the whole measurement depends on
• Light of Day 1, Christological Reading, the Genesis 1:3 treatment
• Second Law of Thermodynamics, the thermal history this radiation records
• Origins and Science, domain hub